Method for preparing improved sulfidic pigments



Patented July 29, 1952 'METHOD FOR PREPARING IMPROVED SULFIDIC PIGMENTS James oBrientgaltimole, Md., assignor to I The Glidden Company, Cleveland, Ohio, at corporation of Ohio No Drawing. Application August 20, 1948,

Serial No. 45,437

This invention relates to improvements 1:; Claims. (01.23-134) methods for making 'zinc and/or cadmium sulfide and sulfoselenide pigments of the extended or unextended types.

Barium sulfide or sodium sulfide solutions have been used extensively for many years in the production of zinc and cadmium pigments byreacting one of the sulfide solutions with soluble zinc and/or cadmium salts to bring, about the precipitation of these latter metals in the form of sulfides. By appropriate selection .of soluble zinc and/or. cadmium salts, and by including selenium inthe sulfide, solutions when desired, the reactions may be. directed to produce pure zincsulfide pigments, cadmium zinc sulfide pigments, pure cadmium sulfide or sulfoselenide pigments,v zinc lithopones, zinc-cadmium lithopones,cadmiumlithopones, and cadmium sulfoselenide lithopones. In theuse of alkali-metal and barium sulfide solutions for producing such types of pigment, it has long been recognized that when such metal sulfide is dissolved in water it hydrolyzes to form the corresponding metal hydrate and sulfhydrate. It has also been long indicated that within the concentration ranges and temperaturesusually employed for reactions of the above type, the ratio of the sulfhydrate to hydrate concentrations is close to but usually less than unity. A few proposals have been made in the past to alter the ratio of sulfhydrate to hydrate in the metal sulfide solutions prior to effecting the strike. but ratio has heretofore been made by employing hydrogen sulfide, by adding barium hydrate or barium sulfhydrate to a barium sulfide solution or by adding sodium hydrate or sodium sulfhydrate to a sodium sulfide solution. In my copending application Serial No. 787,038, filed November 17, 1947, now Patent No. 2,500,958, I have described and claimed a novel method employing refrigeration of the sulfide solution. I have now found, however, that the ratio of sulihydrate to hydrate in such alkali-metal or barium sulfide solutions can be increased appreciably by novel means hereinafter described. I have also found that by so increasingthe ratio, meritorious improvements in the resulting pigments can be achieved, particularly in respect of the tinting strength, color and texture.

Accordingly, it is an object of the invention to such alteration of the provide a novel process for producing improved I sulfide and sulioselenide pigments.

It is a further object to provide a novel process wherein improvements in tinting strength, color and texture of sulfide and sulfoselenide pigments are obtained as a result of increasing the ratio .hydroxyl ,ions from the solution,

of sum/ass to hydrate ih'a ueous saunas and barium sulfide or sulfo selenidesolutions. I

It is a further object to increase the .ra'tio'of sulihydrate to hydrate in aqueous alkali-metal or barium sulfide solutions by precipitating and'removing compounds containing hydroxyl ions from said solutions.

These and other objects will be apparent from the following description of the invention,

Barium sulfide and sodium sulfide hydrolyze according to the following ionic equations when dissolved in water:

The molecular ratio of the sulfhydrate to hydrate in the hydrolysis is indicated here to be 1 to 1. Under some conditions, however, .the molecular ratio may be somewhat less than 1 to 1. I have found, however, that whatever the initial ratio may be as a result of the hydrolysis conditions, by adding to the solution a water-soluble calcium and/or magnesium salt, calcium and/or magnesium hydrates are precipitated and may be filtered off. By so precipitating part of. the the weight ratio of sulfhydrate ions to hydrate ions inthe liquor can be increased quite appreciably; Rreferably,'the precipitate is filtered off as completely as possible so as to provide a. clarified filtrate. The filtrate with its higher ratio of these ions may then be reacted preferably at moderate temperatures below about C. with various soluble zinc and/ or cadmiumsaltsto produce sulfide pigments which manifest improved color, texture and tinting strength. When sulfoselenide pigments are desired, selenium may be dissolved in the sulfide solution which has been treatedqto modify'its SH/Ol-I ratio, and the sulfoselenide solution may then be reacted with ithe' pigment metal salts.

In carrying out my invention 1' may start with any convenient concentration in water of barium or alkali-metal sulfide, from one very'dilute-to one approaching saturation. By adding the said calcium or magnesium salts in small variable amounts a'metathesis occurs and calcium or magnesium hydrates are formed and precipitated, with the result that relatively large percentages of hydroxyl ions can be immobilized whether the resulting precipitate is or isnot removed from the liquor. As indicated, however, in the usual pigment practice, the precipitate is filtered ofi as completely as possible. The precipitates are readily agglomerated in warm liquors and may be filtered off quite completely without undue difiiculty. The sulihydrate ion concentration of the solution being treated is not materially afiected by the treatment so that the filtrate contains most of the sulfhydrate ions wh ch "were originallypresent, -while containing only that gquantity :of barium "or "alkali-metal hydrate which corresponds to the hydroxyl ions remaining in the solution; e. g., in the filtrate after the filtration step has been completed. As a result, the ratio of sulfhydrate tollrydratelions in the solution is increased substantiallydnidirect proportion to the amount of hydroxylions immobilized or removed in thejformof calcium .or magnesium hydroxide. The filtrate is substantially free of calcium or magnesium ions arising from the added salts, since thesmall amounts" of salts added for the purpose of efi'ecting adjustment of the sulfhydrate to hydrate ionseare precipitated quite completely as calcium or maginesium hydroxides, due to the relatively slight "solubility of "these compounds. Magnesium hy- ..droxide"is; more" insoluble than-calcium-hydroxide, softhat magnesium-salts are -preferably"used--if a "solutionalmosticompletely free of "the precipitat- "ing'ications "is "desired. -The metathetical re- "actions which occur are' illustrated'by thefollowing unbalanced schematic reactions:

f'IIn"thesereactions A "represents -*any suitable "innocuous acid-radical. -This acid radical or tanion is "one which when *combined with either *magnesium or calcium produces -=a water-soluble magnesium salt or *a water-=solubleca'lcium salt. "fitiisalsopreferable that the-acid radical beone which yields water-solublesalts of barium: or of ttl eya-lk'ali metals. This preferencewill be apyparent-when it is recognized that if the acid *radic'alyields insol1ible salts of i bariumor of' the alkali metals, then the metathesis not only removes- (DH-ions from the sulfijde sol-ution 'bu'tialso ---reino'ves'some' barium or alkali; metal ions. .3 Sulfate' ions---'andchromate-= ions, for examplep-would y-ield insoluble barium sulfate or barium. chroma te,-and=these insoluble Zbariuma'salts'wouldzbe fil tered-dfi along with the magnesium orccalcium hydrates formed by-the metathesis. asuchr lossxof -la'ariun 1 dons is undesirable, particularly When-the b'ai'ium sulfide :solu'tion' whichis tbeingftreatedzito modify 'its SHZOH =ratio is: intended :foresubsequent use in-preparing'a llithoponeetype .pigm-ent. -Numerous :ciacid radicals Ilf these :desired ichar- ;.:aeteristics .are available :for; use in the: invention, as for example, nitrate, chlorate, perchlorate, lrchlorides bromide, iodide,:sulfide. as derived from e'calciumsulfide, sulfhydrataformate,isobutyrate, eetc. JEorimostpurposes .I;:.prefer to use calcium magnesium -nitrate:"sinoe .I have found [that ssulfrde' solutions which :have :been treated with *zthesemi'trate rsaltszexhibitiacreduced tendency. to crystallize. r indicated-above, :I have "found that Falkali .zme'talzssulfidei solutions and alkali-earth .metal sulfide solutions which have: been treated in the edescribed rmanner. are istable in /.respect 7 to their winoreased sulfhydrateeto hydrate ratios for long .periods 'of timeeattemperatures up -toabout 100 :10. :Stability-rat.rthiszor .lower temperatures :is an r-outstanding :characteristicof the solutions .preylzuaredlgythe present invention-andis in contrast ,fmodified-SH/OHratio may be made in any of the .usual:.manners with the soluble zinc and/or cad- -mium salt solutions. That is, the two solutions may'.be'reacted together simultaneously, the modifiedisulfidesolfi'tion may be added to the soluble pigment-metal salt solution, or the pigment metal salt solution may be added to the modified sulfide solution. Other sequencies may also be used .witho-ut material effect on the improvements afforded by the modified SH/ OH ratio of the sulfide solution. As indicated, it is preferable that the zstrikebeefiected at tempera'turesbelow about 50 "C: ashigher temperatures may-"result" in thedoss of some sulfur in the form 'of hydrogen sulfide. The densities or concentrations ofthe'solubleipigment=metal salt solutions are immaterial but preferablyare' neither more-dilute nor moreconcentrated than is customary and well understood by' those skilled in the-art.

The pH' of thestrike pulp' may" range between about 122 an'dffidepending on"the particular pigment which is-being prepare'd. It 'is generally recognized "in the art that the cadmium' pigments -may be precipitated throughout this entire range, and -it is -also generally "recognized 'tha't'the zinc sulfide ori lithopone pigments 'maybeprecipitated 'atp pHs as low as' about 137. fTheseleetionof appropriatepH valuesis"accordingly well-within the ability'of oneskilled in the-art, "due regard-being given to *reco'gnized'efiects of" pH onthe pigmentary qualities of the particular'type of-pigment* beingjprecipitated and treated. Iparticu- 'larly prefer, however; to secure-a final pH of about 630 in eifecting the strikes for any of the zinc or zinc-cadmium pigmentsmentioned hereinaib'ove. The precipitates obtained fromthe strikesare generally -dried, and then "calcined under conditions of temperature-and atmosphere well known to those'skilled in the art. Howeverpsomeprecipitatesysuch as the cadmium sulfide yellow, need not'be-calcined tobesui'ta'ble for use-as pigments. Accordinglydt'willbe understood that the calcining step maybe an optional step in "a process for making pigments of improvedtquality in accordance'with theprinciples of this invention.

The following examples illustrate the principles of my invention:

"Example 1 .A stock calcium nitrate solution-was prepared ibyudissolving 340. grams of calcium hydroxide in .550 ml. nitric acid-andi500, mlnwater. The solution was made neutral to methyl orange. and was filtered. 'Thefiltrate had a'density of 2416'B. at 33 C.

A' barium "sulfidesolution having a density "of 17"B. at 52 C. was treated to modify its S H/OH ratio, 'byad'ding 57.5 ml.'of thestock calcium nitrate solution to 250 ml. of the barium -=sulfide solution; heating the mixed *solutions'to -5'0 'C. and filtering off the precipitate of calciumfhydrate. The filtrate had a densityrof 15 v'IB.at K50": .C.=: and an SI-I/OI-I :ratio of 7.3.

Example 2 Two and one-half liters ofa bariumzsul-fide solution havingadensity of -2-1-B. at 408C. were "treated with..400. ml. ofa calcium chl0ride.soludensity of 20 tion which was neutral to methyl orange and had a density of 18.5 B. at 37 C. The mixture of solutions was warmed to 50 C. and the precipitate of calcium hydroxide which formed was filtered off. The filtrate was a barium sulfide solution having a density of 188 B. at 32 C. and an SI-l/ OH ratio of 1.77. When the treatment was repeated except that the mixtureof solutions was heated to 100 C. before filtering off the calcium hydroxide, a filtrate was obtained having an SH/OH ratio of 1.86. The higher ratio results from the decreased solubility of calcium hydroxide at the higher temperature.

Example 3 When the filtrate was heated to 100 0., its SH/Oi-I ratio remained unchanged, indicating its stability at high temperatures.

Example 4 To 250 ml. of a barium sulfide solution having a Be. at 50 C. was added 49.5 ml. of the stock calcium nitrate of Example 1 precipitate of calcium hydrate was filtered off. The filtrate was a barium sulfide solution having v a density of.17 Be. at 50 Gland an Sl-l/OH ratio To the sodium sulfide solution was added 600 ml. of the stock calcium nitrate solution of Example i. The mixture of solutions was heated to 50 C.'and the precipitated calcium hydrate was filtered oil. The filtrate was a sodium sulfide solution having an SH/OH ratio of 2.

7, Example 6 v A stock magnesium nitrate solution was prepared by dissolving 650 grams of technical magnesium carbonate in 2 liters of water and 845 ml. of 42B. nitric acid. Thesolution was made neuwas filtered ofi. The filtrate was a barium sulfide solution having a density of 17 B. at 50 C. and an'SH/OI-I ratio of 2.05.

The mixture or solutions was heated to 50 C. and the I from an initial furnace temperature of 368 C. to

tral to methyl orange and was then filtered. The filtrate was a magnesium nitrate solution having a density of 23.6 B. at 36 C.v

To 2 liters of a barium sulfide solution having I a density of 17.'l Be. at 36 C. was added 210 ml.

ofv the stock magnesium nitrate solution. The mixture of solutions was heated to 50 C. and the precipitated magnesium hydrate was filtered off.

The filtrate was a barium sulfide solution haying a density of 164 B. at 22 C. and'an SH/OH ratio of 2.

Example 7 A pure yellow primrose pigment was prepared by mixing 250 ml. of a cadmium nitrate solution (density, Be. at 29 C.) with 62.5 ml. of zinc nitrate solution (density, 32 B. at35 C.) and slowly adding to the mixture of solutions 1056 ml.

of the barium sulfide solution prepared in Examdried at about 150 C. The dry cake was then calcined from an initial temperature of 368 C. to a final temperature of 588 C., and was quenched in water from the final temperature. The quenched pigment was again dried. The drypigment had a color slightly darker than the Primrose Standard, and had a. tinting strength of Example 8 A pureyellow pigment was prepared by slowly adding I20 ml. of the barium sulfide solution prepared in Example 3, to 250 ml. of a cadmium nitrate solution (density, 35 B. at 20 C.) to a final pH of 5.5 at a final temperature of 30 C. The slurry was filtered and the filter cake was reslurried in 2 liters of water at 0., being agitated therein for one hour. The reslurry was filtered and the filter cake was dried at about 150 C. The dry cake was calcined from an initial temperature of 368 C. to a finaltemperature of 538 C., then .quenched in water, filtered and dried. The pigment had a color close to the Orange Standard and a tinting strength of 335%.

Example 9 A pure yellow'pri'mrose pigment was prepared by first mixing 250ml. ofa cadmium nitrate solution (density 353 B. at 23 C.) with 62.5 inlet a zinc nitratesolution (density 32.3 Be. at 29 C.) and then slowly addingthis mixture of solutions to 985 ml. of the barium sulfide solution prepared in Example 6, to a final pH of 5.5 and a final temperature of 30 C. The slurry was filtered and the filter cakewas reslurried in 2 liters of water at 50 C-., being agitated therein for on hour. The rcslurry was filtered, andthe filter cake was dried at about 150C. The dry filter cake was calcined a final temperature of 588 C., quenched in water, filtered and .dried. The pigment had a color lighter than the Primrose Standard and a, tinting strength of 235%. v

- Example 10 A pure red pigment was prepared by reaction of a cadmium nitrate solution with a barium sulfide solution of modified SH/OH ratio having selenium dissolved in it. The barium sulfide solution was modified in its SI-I/OH ratio by treatment with calcium nitrate solution as described above to produce a filtrate having a density of'l 4. 2-B. at 265 C. and an SH/OH ratio of 2.82. Two liters of the filtrate were measured out and 30.25 grams of selenium were dissolved therein. Onehalf liter of a cadmium nitrat solution (density 35P-Be. at 26 C.) was slowly added to the barium sulfide-selenium solution tofafinal pH of 2.1 and m2 liters of water; at 50 C. and-was agitated therein for one hour. The reslurry was filtered and the filter cake was dried at about 150 C. To

Y the dried cake was added and mixed 15% of amy monium nitrate, and the mixture was calcined in an open crucible to about600 C.,'then promptly quenched in water, filteredand dried. The dry v pigment had a color slightly. lighter than the ple 4, having an SH/OI-I ratio of 3.04. The pH was adjusted to 5.3 and the slurry was filtered.

The filter cake was reslurried in 2 liters of water at 50 C. and was therein agitatedfor one hour.

The reslurry was filtered, and the filter cake was Medium Red Standard-and had a tinting strength 0f230%. '5

. Example 11 A zinc lithopone pigment'was prepared by reaction of 400 ml. of zinc sulfate (density, 263 B. at 44 C.) with'1315 ml. of a barium sulfide solution (density 1 8.5 B. at 32 C.) having an 'SHYOH ratio or 3.04; The modified ratio was obfinal pigment produce o'fi-colors.

-. tained bytreating the barium'sulfide.solution with -.-calcium nitrate and filtering. off the precipitated calcium hydrate. In making the pigment strike, the barium sulfide solution'was slowlyaddedto the.zincjsulfate solution to a finallyeadjusted pI-I -of and a. -final.temperature;of-.-29 C. The resulting slurry was :filtered; and thefilter cake wasreslurried in two liters of water at 50 C., being-agitated therein for one hour. The reslurry was filtered and the filter cake was dried at about 150 C. The :dry pigment was-calcined between an initial "temperaturebf 1368" :C.:anda final temperature :of 714 C., and "was then f promptly 1qu'enched' inwater, filtered and dried. The pig- "ment-was -slightly cit-color but had a tinting "strength of 113% as-compared-with 100% for a commercial batch prepared "conventionally without treatment of th barium sulfide solution to hating ions. The following exampleillustrates 'thisgresult.

Example..;l 2

A yellow lithopone pigment-was "prepared by slowly adding 1100 1 ml. of the barium "sulfide solution prepared in'Example 3 above'to 500 ml. of'a cadmium'sulfate solution (density 35 Beat 20 C.) contaminated-with 165 gram of nickel *sulfate (NiSO'4-6H2O). The resulting slurry had a final temperature of 29 C.- and itspI-I was adjusted to 1.2. The slurry was filtered, given-a filtrate which was about .175 N in acidity. The filter cake was reslurried in 2 liters of water at 50 C. with agitation therein for one hour. The 'reslurry was filtered andthe filter cake was dried atabout 150 C. The dried cake was'calcined from an initial furnace temperature of 368 C. to a final temperature-of 536 0.; then wasquenched in'water, filtered and dried. The dry pigment was abrightgolden-yelbw with 'a" tinting strength of 140%. a

From the foregoing description of the invention, itgwill be apparent thatthe principles of the invention may be applied-in a variety of ways to secure the advantages which flow from stable Afurther 'embodiment ofth'e invention is yet to be described, wherein calcium sulfhydrate'is the 'soluble second -group metal salt which is used to the resulting leach liquor is aibariumsulfide solution whose .SH/OH ratio is higher. than would be obtainedif gypsumhad not been .added to the sulfide solutions-having increased SH/OH ratios.

When the calcined mixture is subsequently leached with water,

8 barytes-prior to-calcinationvof the latter. The reactions involved are:

LCaSOi+ 4.0 heat ,OaS 40.0 "20:18 +2mo Ea H 2-+ oa oH),1

2139.6 2320 insfin liawfi), 312ml? i-fa sfi)2 (mount hash), Thus calcium sulfate is used to introduce calcium sulfide into the calcined barytes, and when the calcined product is leached to extract its barium sulfide, the calciumsulfide therein is also leached'and is hydrolyzed to calcium sulfhydrate and calcium hydroxide. The latter hydroxide is largely insoluble and remains behind with the gangue of the barytes ore. ,The barium sulfide also hydrolyzes during the leaching operation to yield barium sulfhydrate and barium hydroxide, but the latter hydroxidereacts with the-calcium sulfhydrate of the leach liquor to produce the 'soluble'barium sulfhydrate and the relatively insoluble calcium hydroxide. When the leach liquor is finally clarified to produce a barium sulfide solution whichis to be used in "pigment strikes, the two calcium hydroxide precipitates of the foregoing reactions are rejected along with other insolubles, gangue, etc. Accordingly, the principles of this invention are here, used in such manner that the processing stepsemployedin producing a barium sulfide solution from barytes are also effective in carrying out eachof the steps of the present invention which are necessary to effect modification of the SH/OH ratio -of the barium; sulfide solution.

Various other embodiments-of the principles of this invention will be apparent to those skilled in the art.

In the following claims, the term sulfidic pigment is used to refer to all conventional zinc and/or cadmium sulfide or 'sulfo-selenide pigments, such as zinc sulfide pigments, zinc lithopone pigments, cadmium-zinc sulfide pigments, cadmium sulfo-selenide pigments, cadmiumsulfied lithopones, cadmium sulfo-selenide lithopones, and cadmium sulfide pigments.

In the foregoing examples and elsewhere in'the specification, tinting strength values refer to values obtained by a tinting strength test now to be described. In this test T 5 gram of the standard 'color is 'mixed with 2 grams of a standard grade'of lithopone, and with-1 6 ml. of pale'linseed 'oil, the mixing being eifected on a'glass plate by means ofa spatula. The resultingpaste is then'mulled by means of forty forward and :forty backward strokes of the muller within'a space of about 12 inches x 12 inches on the glass plate. After one such mullingthe paste is gathered into a pile and is then mulled a'second time with forty more-backward and forward strokes of the muller. On another glass plate, the pigment whose strength is to be determined '(called the unknown) is similarly mixed and mulled with linseed oil and standard lithopone, gram 'of the unknown being mixed and mulledwith that weighed amount of the standard lithopone which iseStimated as necessary to give the same tint as that of the samplemade with the. standard color. The twoglass plates are then placed side by side and the tints compared. Ifthe same tints have not been obtained on. both plates, then a new plate is made up from the unknown using moreor less standard lithopone as is requiredto obtain thesame tint. 'When finally the unknown and standardfplates have the same tint, thetinting strength is calculated on the basis of the 7 percentage 'of lithopone in the unknownto. the

lithopone in the standard. Thus if 1.8 grams of lithopone was mixed with the unknown, the tinting strength is 90% (1.8/2.0 x 100), While if 2.2 grams of lithopone was used with the unknown, the unknown has a tinting strength of 110% (2.2/2.0 x 100).

Having described my invention, what I claim as new is:

1. The method of making an improved sulfidic pigment which comprises the steps of: providing an aqueous solution of a metal sulfide selected from the group consisting of alkali metal sulfides and barium sulfide; reacting said metal sulfide solution with a water-soluble salt of a secondgroup metal selected from the group consisting of calcium and magnesium to precipitate an insoluble hydrate of the said second-group metal and thereby to establish in said sulfide solution an SH/OH ratio in excess of 1 to 1; separating said insoluble hydrate from its mother liquor; striking the resulting mother liquor having an SI-I/OI-I ratio in excess of 1 to 1 with an aqueous solution of a water-soluble salt of at least one pigment metal selected from the group consisting of zinc and cadmium, thereby to produce a sulfidic precipitate of the said pigment metal; and separating said sulfidic precipitate from the liquors of the strike slurry.

2. The method as claimed in claim 1 which includes the further step of calcining said sulfidic precipitate under conditions which convert it to a pigment.

3. The method as claimed in claim 1 wherein the aqueous solution of said pigment-metal salt contains minor amounts of heavy metals having atomic numbers between 25 and 28, inclusive.

4. The method as claimed in claim 1 wherein the metal sulfide solution is a solution of barium sulfide.

5. The method as claimed in claim 4 wherein the salt of second-group metal is one whose anion yields a water-soluble salt of barium.

6. The method as claimed in claim 5 wherein the aqueous solution of said pigment-metal salt is contaminated with minor amounts of heavy metals having atomic numbers between 25 and 28, inclusive.

'7. The method of making an improved cadmium sulfoselenide pigment which comprises the steps of: providing an aqueous solution of a metal sulfide selected from the group consisting of alkali metal sulfides and barium sulfide; reacting said metal sulfide solution with a water-soluble salt of a second-group metal selected from the group consisting of calcium and magnesium to precipitate an insoluble hydrate of the said second-group metal; separating at least a part of said insoluble hydrate from its mother liquor thereby to establish in said mother liquor an SH/OH ratio in excess of l to 1; dissolving selenium in the resulting mother liquor to make a strike solution; striking the said strike solution with an aqueous solution of cadmium salt, thereby to produce a cadmium sulfoselenide precipitate; separating the liquors of the reaction mass from precipitated matter thereof; and calcining said precipitate of matter to convert it. to pigment.

8. The method as claimed in claim 7 wherein the aqueous solution of said cadmium salt con-" tains minor amounts of heavy metals having atomic numbers between 25 and 28 inclusive.

9. The method as claimed in claim 7 wherein the metal sulfide solution is a solution of barium sulfide.

10. The method as claimed in claim 9 wherein the cadmium salt is one whose anion yields a water-soluble salt of barium.

11. The method as claimed in claim 10 wherein the aqueous solution of said cadmium salt contains minor amounts of heavy metals having atomic numbers between 25 and 28, inclusive.

12. The method of preparing an improved barium sulfide solution from barium sulfate, which comprises the steps of: mixing a small amount of calcium sulfate with the barium sulfate; calcining the mixture under reducing conditions effective to reduce the sulfates to sulfides; dissolving at least a part of the resulting sulfides in water; and separating the resulting solution from any remaining undissolved matter.

13. The method of making an improved sulfidic pigment which comprises the steps of: mixing a small amount of calcium sulfate with barytes ore; calcining the mixture under reducing conditions' effective to reduce the calcium sulfate and barytes to sulfides; leaching the calcined mixture with water to extract soluble sulfides; separating the leach liquor from any undissolved residues and entrained solid matter, thereby to clarify said liquor; striking said clarified liquor with an aqueous solution of a water-soluble salt of at least one pigment metal selected from the group consisting of zinc and cadmium; thereby to produce a sulfidic precipitate of said pigment metal; and separating said sulfidic precipitate from the liquors of the strike slurry.

14. The method as claimed in claim 13 which includes the further step of calcining said sulfidic precipitate under conditions which convert it to a pigment.

15. The method as claimed in claim 13 wherein the aqueous pigment-metal salt solution is contaminated with minor amounts of heavy metals having atomic numbers between 25 and 28, in-

,clusive.

16. The method as claimed in claim 15 wherein the cadmium salt is one whose anion yields a water-soluble salt of barium.

17. The method as claimed in claim 15 Where- 1 in said clarified liquor; striking th resulting liquor with an aqueous solution of a cadmium salt, thereby to produce a cadmium sulfoselenide precipitate separating the liquors of the strike slurry from the precipitated matter thereof; and calcining the precipitated matter under conditions which convert it to pigment.

JAMES J. OBRIEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,776,029 Howard Sept. 16, 1930 1,936,849 Mertes q. Nov. 28, 1933 2,030,887 Mitchell Feb. 18, 1936 2,050,802 Mitchell Aug. 11, 1936 

1. THE METHOD OF MAKING AN IMPROVED SULFIDIC PIGMENT WHICH COMPRISES THE STEPS OF: PROVIDING AN AQUEOUS SOLUTION OF A METAL SULFIDE SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL SULFIDES AND BARIUM SULFIDE; REACTING SAID METAL SULFIDE SOLUTION WITH A WATER-SOLUBLE SALT OF A SECONDGROUP METAL SELECTED FROM THE GROUP CONSISTING OF CALCIUM AND MAGNESIUM TO PRECIPITATE AN INSOLUBLE HYDRATE OF THE SAID SECOND-GROUP METAL AND THEREBY TO ESTABLISH IN SAID SULFIDE SOLUTION AN SH/OH RATIO IN EXCESS OF 1 TO 1; SEPARATING SAID INSOLUBLE HYDRATE FROM ITS MOTHER LIQUID; STRIKING THE RESULTING MOTHER LIQUID HAVING AN SH/OH RATIO IN EXCESS OF 1 TO 1 WITH AN AQUEOUS SOLUTION OF A WATER-SOLUBLE SALT OF AT LEAST ONE PIGMENT METAL SELECTED FROM THE GROUP CONSISTING OF ZINC AND CADMIUM, THEREBY TO PRODUCE A SULFIDIC PRECIPITATE OF THE SAID PIGMENT METAL; AND SEPARATING SAID SULFIDIC PRECIPITATE FROM THE LIQUORS OF THE STRIKE SLURRY. 